Rollup merge of #49555 - nox:inline-into-boxed, r=alexcrichton

Inline most of the code paths for conversions with boxed slices

This helps with the specific problem described in #49541, obviously without making any large change to how inlining works in the general case.

Everything involved in the conversions is made `#[inline]`, except for the `<Vec<T>>::into_boxed_slice` entry point which is made `#[inline(always)]` after checking that duplicating the function mentioned in the issue prevented its inlining if I only annotate it with
`#[inline]`.

For the record, that function was:

```rust
pub fn foo() -> Box<[u8]> {
    vec![0].into_boxed_slice()
}
```

To help the inliner's job, we also hoist a `self.capacity() != self.len` check in `<Vec<T>>::shrink_to_fit` and mark it as `#[inline]` too.

src/liballoc/string.rs

@@ -1586,6 +1586,7 @@ impl String {
     /// let b = s.into_boxed_str();
     /// ```
     #[stable(feature = "box_str", since = "1.4.0")]
+    #[inline]
     pub fn into_boxed_str(self) -> Box<str> {
         let slice = self.vec.into_boxed_slice();
         unsafe { from_boxed_utf8_unchecked(slice) }